In recent years, the market of the touch panel type image display device, such as smartphones and tablets has grown rapidly, and the image display device is required to be furnished with design properties, fashionability, and decorativeness, and thus the thinning, weight saving, and variation in color of the image display device has progressed.
For example, in the surface protective panel of an image display device, a plastic material such as an acrylic resin plate or a polycarbonate plate is used together with tempered glass or a member is used which is printed in white together with common black as the back printing of the peripheral portion other than the viewable aperture surface portion of the surface protective panel.
Moreover, in the touch panel, a plastic film sensor is used together with a glass sensor, or a touch on lens (TOL) member is used in which the touch panel function is integrated with the surface protective panel, furthermore, an on-cell or in-cell type member is used in which the touch panel function is integrated with the surface protective panel.
In addition, these surface protective panel, touch panel, and the image display panel (collectively, also referred to as the “components for image display device”) are combined to form the touch panel type image display device.
Furthermore, a structure is general in which the gaps between the respective components for image display device are filled with a transparent resin in order to further improve the image visibility.
For example, a liquid adhesive, a thermoplastic adhesive sheet material, an adhesive sheet material or the like is known as the resin material to fill the gap.
In the case of liquid adhesive, resin sealing is possible since it freely flows through the gap, but there is a problem that it is difficult to obtain in-plane thickness accuracy in the viewable aperture surface portion and ultraviolet light does not reach the printing back portion of the protective panel when the liquid adhesive is cured by irradiating with ultraviolet light so as to remain in an uncured state and to contaminate the electrodes or the wires and also there is a concern that the uncured liquid resin attacks the plastic member in the image display device with time so as to cause solvent cracking and peeling between the members occurs due to the curing shrinkage in the cured portion.
In addition, in the case of the thermoplastic adhesive sheet material, it is possible to seal the gap by heating and melting the sheet material, but there is a concern that the resin overflows during heating and melting or the residual stress or the warp occurs due to heating linear expansion difference, a decrease in cycle time due to the heating process occurs, and a problem of temperature management at the time of storage or transportation in the case of the material to melt at a low temperature rises.
In that respect, it is possible to easily bond the sheet material at room temperature, the in-plane dimensional accuracy is also significantly favorable, and the cycle time is also short to be excellent in mass productivity in the case of the adhesive sheet material.
An adhesive sheet to bond the components for image display device to each other is disclosed in Patent Documents 1 and 2.